The present invention generally relates to power generation systems and, more particularly, to systems and methods for connecting the rotating rectifier on a generator or motor.
The electrical machines used in aerospace power generation systems operate at very high speed. It is therefore desirable to make the generators used in these systems brushless, to avoid frequent maintenance or replacement. Brushless generators also help in controlling the noise made by brushes. Moreover, transients may be introduced by the brushes into the current may interfere with the proper operation of electrical loads.
A typical brushless generator has three distinct generating systems—a permanent magnet generator, an exciter and a main generator. The permanent magnet generator includes rotatable permanent magnets for establishing a magnetic field that induces alternating current that is typically fed to a regulator or a control device. The regulator or control device outputs a direct current signal and stationary field windings of the exciter are electrically coupled to receive the direct current signal output from the regulator or control device and develop a magnetic field in the exciter. The exciter may also include exciter armature windings mounted on the generator rotor, which rotate within this magnetic field.
The magnetic field in the exciter is in turn employed to induce an even higher level of current. Typically, the exciter armature windings are wound such that the induced alternating current signal is a three-phase alternating current signal.
Rectifier circuits that rotate with the exciter armature windings rectify the three-phase alternating current signal induced in the exciter armature windings. The direct current output from the rectifiers is provided to the main generator. Typically, conversion from three phase alternating current to direct current in a generator is accomplished using a full wave bridge as disclosed in U.S. patent application no. 20070108854 by Osborne et al.
In a typical full wave bridge connection, an extension of the flat copper wire is brought into the axial vicinity of the exciter from the main coil. Six separate stranded leads may be brazed in a common connection to the flat wire. These leads are then brought out radially around the exciter rotor core spokes as flying leads and terminated on the diodes. This results in a complex set of connections linking six diodes to two main rotor leads of the generator, which is susceptible to damage from the high centrifugal loading present on a rotor, especially, in aircraft applications that typically rotate at relatively high speeds (e.g., 24,000 r.p.m. or greater). The complex set of connections is also susceptible to shorting and must be electrically insulated to prevent shorting of the electrical machine.
The main generator includes rotating field windings and stationary armature windings. The rotating field windings are electrically coupled to receive the DC current from the rectifiers and develop a magnetic field that rotates with the rotor. This rotating magnetic field further induces a three-phase AC current in the stationary armature windings. This three-phase AC current is then provided to a load.
Typically in a brushless generator, in order to generate a suitable magnetic field in the rotor, it is necessary to utilize direct current as opposed to alternating current. Since the output of the exciter is an alternating current, it is desirable that this alternating current must be rectified by a rectifier to direct current. To avoid resorting to brushes, it is desirable that the rectifier assembly interconnecting the exciter armature windings and the main generator rotating field winding be carried by the rotor of the generator. It is necessary that such rectifier should also be capable of withstanding high centrifugal loading. U.S. Pat. Nos. 4,570,094; 4,603,344 and 4,628,219 disclose examples of known rotating rectifier assemblies.
A separate wound resistor in its own unique bobbin assembly may be glued to the shaft and the end of the resistor wire terminates in the same common braze as the diode leads. All these operations are carried out at the assembly level of the rotor and any mistake can lead to reworking or scrapping of the device.
U.S. Pat. No. 4,570,094 issued to Trommer, discloses a rectifier assembly where the diode wafers are angularly spaced in two parallel planes. The diodes are sandwiched between conductor plates which are held under compression by a biasing device to assure good electrical contact between the various components. But it relied on clamping forces in order to maintain electrical contact. Similarly U.S. Pat. No. 4,603,344 issued to Trommer relies on means for applying a compressive pressure to bring elements and wafers into good thermal and electrical contact. It also relied on applying a compressive pressure in order to maintain thermal and electrical contact.
U.S. Pat. No. 4,581,695 issued to Hoppe, discloses rectifier assembly that included two side-by-side conductive blocks, each having three outwardly facing diode receiving surfaces. A diode is positioned on each surface and three phase bars are used to connect aligned diodes for each of the two blocks. A metal housing is shrink fitted using thermograding to hold the structure in assembled relation. Therefore, this assembly could not be repaired without destroying the housing.
As can be seen, there is a need for a system and method for connecting main rotor winding to the exciter diodes that can withstand high centrifugal loads present on a rotor and provides an impeccable connection to preventing shortening of machine, can be tested and assembled outside before inclusion to the rotor and does not add size and cost of the power generating system.